.\"	$OpenBSD: dup.2,v 1.15 2013/03/31 08:34:27 guenther Exp $
.\"	$NetBSD: dup.2,v 1.4 1995/02/27 12:32:21 cgd Exp $
.\"
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.\"     @(#)dup.2	8.1 (Berkeley) 6/4/93
.\"
.Dd $Mdocdate: March 31 2013 $
.Dt DUP 2
.Os
.Sh NAME
.Nm dup ,
.Nm dup2
.Nd duplicate an existing file descriptor
.Sh SYNOPSIS
.Fd #include <unistd.h>
.Ft int
.Fn dup "int oldd"
.Ft int
.Fn dup2 "int oldd" "int newd"
.Sh DESCRIPTION
.Fn dup
duplicates an existing object descriptor and returns its value to
the calling process
.Fa ( newd
=
.Fn dup oldd ) .
The argument
.Fa oldd
is a small non-negative integer index in the per-process descriptor table.
The value must be less than the size of the table, which is returned by
.Xr getdtablesize 3 .
The new descriptor returned by the call is the lowest numbered descriptor
currently not in use by the process.
.Pp
The object referenced by the descriptor does not distinguish between
.Fa oldd
and
.Fa newd
in any way.
Thus if
.Fa newd
and
.Fa oldd
are duplicate references to an open
file,
.Xr read 2 ,
.Xr write 2
and
.Xr lseek 2
calls all move a single pointer into the file,
and append mode, non-blocking I/O and asynchronous I/O options
are shared between the references.
If a separate pointer into the file is desired, a different
object reference to the file must be obtained by issuing an
additional
.Xr open 2
call.
The close-on-exec flag on the new file descriptor is unset.
.Pp
In
.Fn dup2 ,
the value of the new descriptor
.Fa newd
is specified.
If this descriptor is already in use, it is first deallocated as if a
.Xr close 2
call had been done first.
When
.Fa newd
equals
.Fa oldd ,
.Fn dup2
just returns without affecting the close-on-exec flag.
.Sh RETURN VALUES
The value \-1 is returned if an error occurs in either call.
The external variable
.Va errno
indicates the cause of the error.
.Sh ERRORS
.Fn dup
will fail if:
.Bl -tag -width Er
.It Bq Er EBADF
.Fa oldd
is not a valid active descriptor.
.It Bq Er EMFILE
Too many descriptors are active.
.El
.Pp
.Fn dup2
will fail if:
.Bl -tag -width Er
.It Bq Er EBADF
.Fa oldd
is not a valid active descriptor or
.Fa newd
is negative or greater than or equal to the process's
.Dv RLIMIT_NOFILE
limit.
.It Bq Er EINTR
An interrupt was received.
.It Bq Er EIO
An I/O error occurred while writing to the file system.
.El
.Sh SEE ALSO
.Xr accept 2 ,
.Xr close 2 ,
.Xr fcntl 2 ,
.Xr getrlimit 2 ,
.Xr open 2 ,
.Xr pipe 2 ,
.Xr socket 2 ,
.Xr socketpair 2 ,
.Xr getdtablesize 3
.Sh STANDARDS
.Fn dup
and
.Fn dup2
are expected to conform to
.St -p1003.1-2008 .